1. Field of the Invention:
The present invention relates to a navigation system for outputting information required to reach a destination.
2. Description of the Related Art:
Recently, navigation systems for vehicles have been developed which are designed to output on a display various types of information required for a driver who is unfamiliar with a particular area to reach a destination and thereby guide the driver to the destination.
Conventional navigation systems fall into two types: the location method and fixed route method.
The location method was proposed in, for example, the specification of Japanese Patent Laid-Open No. 58-115600. In this method, a map stored in an external memory is read out and is stored in an image memory by an arithmetic and logic unit constituted by a microcomputer. Also, the arithmetic and logic unit calculates the locus of a vehicle using data input from azimuth and distance sensors, and displays the map and the locus of the vehicle on a display.
Japanese Patent Laid-Open No. 62-142216 discloses the fixed route type navigation system. In this method, a course that is to be followed from a starting point to a destination is set beforehand, and the driver is guided in accordance with the course set. The next intersection along the course where the vehicle is to turn is indicated in this method by numerically or graphically displaying the distance remaining before that intersection is reached. So, the driver looks at the course indicated on the map and knows the intersection where he should turn. Alternatively, the driver looks at the numerals or graph displayed to ascertain the distance remaining to that intersection, and thus knows the intersection where the vehicle should be turned.
In both the location method and the fixed route method, the information required to guide a vehicle to a destination such as map data or intersection data is stored in one external memory such as a floppy disk, a CD-ROM, or a magnetic tape. Guided navigation to a plurality of areas (e.g., to Nagoya and Kyoto) is made possible by using corresponding external memories which are switched with each other as required.
In both of the above-described conventional navigation systems in which the information required to guide a vehicle to one area, e.g., Kyoto, such as map data or intersection data, is stored in the external memory beforehand, not all the information is stored in the external memory. Also, it is very difficult to store all the information in the external memory, both in terms of storage capacity and the calculation ability of the central processing unit. Even if all the necessary information is input in the memory, a large amount of information makes selection of a destination very difficult. In both of these navigation systems, it is impossible to guide a vehicle if the memory does not contain the information on a desired starting point or destination beforehand. In that case, a driver has to go to a nearby starting point which is stored in the memory without the assistance of the navigation system. He also has to go to his true destination by himself after he has reached the destination input in the memory beforehand.
Further, modification, addition and deletion of the information stored in an external memory such as map data or intersection data requires a great deal of time and expense.
The above-described location method has its own problem in that, in a case where it is necessary to turn at an intersection indicated on a map displayed on the display device, it may be impossible for a driver to identify an intersection where the vehicle is approaching with the one indicated on the map, due to errors accumulated in the measured running distance or direction. The driver may pass an intersection where he should turn, or the present position on the map may not correspond with a road on the map.
In the fixed route method, since the course to be followed from a starting point to a destination is set beforehand and the vehicle is guided in accordance with the course set, it is difficult to change the destination during travel. Further, if, for example, a wrong turning is made and the driver deviates from the original course, it is impossible to continue travelling under the guidance of the navigation system unless the vehicle can be returned to the course originally set. Furthermore, it is necessary to judge whether a vehicle has passed a predetermined intersection as planned on the basis of running distance data prepared by a distance sensor or data representing right or left turns which is obtained through a steering sensor. In consequence, any detection errors which occur during actual travel may cause misjudgement.
In order to obviate these problems, the present inventors filed an application in respect of a navigation system for outputting information required to guide a vehicle to a destination at a plurality of points (in Japan under Japanese patent application No. 62-307805 and in the U.S.A. on Oct. 20, 1988, as U.S. Ser. No. 07/260,213). In this system, the coordinates of a plurality of points are set and the information required to reach the destination is calculated at each of these points. This method (hereinafter referred to as the coordinate origin method) has the advantage that it is possible to change a destination during travel and that the course followed to a destination can be searched relative to any of the plural points. However, as stated above, it is difficult to input all the information required to guide a vehicle to a particular destination. It is therefore essential to decide properly which information is to be stored and how the stored information is to be used.